1. Field of the Invention
The present invention relates, generally, to a method for influencing the level of a radio-frequency transmitted signal in a base station in a fixed radio network and, more particularly, to such a method wherein the level of the transmitted signal is first measured in a transmitting/receiving unit and then measured in a subsequent antenna transmitting/receiving unit such that a level difference is determined and the level is then influenced so as to approximately maintain a predetermined level difference.
2. Description of the Prior Art
In order to produce a fixed radio network, radio base stations are provided to which network termination devices are connected without wires. A network termination device represents the termination of a communications network (for example, a public or private communications network). The network termination device provides an interface (for example, an analog or digital telephone or ISDN interface) to which communications terminals (for example, analog or digital telephone terminals or ISDN communications terminals) are connected. The information to be transmitted from or to the communications terminals is transmitted via radio signals, which are formed in accordance with either the standardized DECT transmission method or the CDMA transmission method, via the radio path between the network termination devices and the radio base stations.
In a radio base station, a radio-frequency signal which is to be transmitted is formed, for example, in accordance with the CDMA transmission method. In order to avoid considerable attenuation resulting from transmission via a cable, in particular a coaxial cable, the transmitted signal is transmitted at a lower level or at a lower power via a cable to an antenna transmitting/receiving unit. The antenna transmitting/receiving unit contains a line amplifier and is connected to an antenna which transmits the radio-frequency transmitted signal as a radio signal. The antenna transmitting/receiving unit is generally fitted at the upper end of an antenna mast close to the antenna. A radio signal is received in the network termination device, and the radio-frequency received signal which is received is used to derive the transmitted information. The antenna transmitting/receiving unit also contains a low-noise amplifier for a received signal. The received signal is passed directly from the antenna, if the transmitting/receiving unit and the antenna transmitting/receiving unit are separate, or via a transmission/reception diplexer in the case of a joint transmitting/receiving antenna, to the amplifier.
The power of the transmitted radio signal or the level of the radio-frequency transmitted signal is, in this case, influenced significantly not only by the level change caused, for example in the case of CDMA transmission method, by the transmission method which is used, but also by the antenna transmitting/receiving unit and by the RF cable or line which leads to the radio base station. Depending on the nature and length of the cable which is used, the level of the radio-frequency transmitted signal may have widely differing level values. Furthermore, in widely different temperature conditions, for example direct solar radiation during the day and low nighttime temperatures, the antenna transmitting/receiving unit will amplify the radio-frequency transmitted signal to considerably different extents and, thus, will additionally influence the level of the radio signal. Furthermore, the level of the received signal at the radio base station is influenced. Particularly in the case of the CDMA transmission method, precise control of the level of the radio-frequency transmitted signal represents a major function which is used to achieve optimum reception conditions in the network termination devices and minimum interference in the further network termination devices. Since level fluctuations in the received radio signal, particularly in radio networks using the CDMA transmission method, can lead to different reception power levels in the network termination devices as will as to operation which is subject to interference, the radio signal which is received there or the radio-frequency transmitted signal in the radio base station should be at a level which is predetermined as much as possible.
An object to which the present invention is directed, therefore to refine a radio base station and the transmission path in a radio base station in such a manner that a radio signal is transmitted approximately at the intended level by the antenna, with transmission-method-dependent level changes having to be ensured.
The major aspect of the method of to the present invention ensures that the level of the transmitted signal which is passed to it is measured in the transmitting/receiving unit, and the level of the transmitted signal which is transmitted via the cable is measured in the antenna transmitting/receiving unit. The level difference between the measured levels is then determined and compared with a predetermined level difference, and the level of the transmitted signal is influenced in such a manner that the level difference which is determined is maintained approximately at the predetermined level difference. In this way, the overall gain of the arrangement formed by the transmitting/receiving unit, the cable and the antenna transmitting/receiving unit is set to a predetermined value or gain factor, and this setting is monitored and, if necessary, readjusted.
The comparison of the level differences and the influencing of the transmitted signal can be carried out in the transmitting/receiving unit or, alternatively, in the antenna transmitting/receiving unit, with the measured levels being transmitted to the transmitting/receiving unit or to the antenna transmitting/receiving unit. A major advantage of the method of the present invention is that the level of transmitted signal at the antenna is kept approximately at a value which is dependent on the transmission method, irrespective of the length and transmission characteristicsxe2x80x94particularly, attenuation) of the cable between the transmitting/receiving unit, in which case the different transmission characteristics are caused by different cable types and by environmental influences such as severe temperature fluctuations. The transmitted signal level control according to the present invention allows optimum reception conditions to be ensured in the network termination devices.
According to one advantageous embodiment of the method of the present invention, the level of the transmitted signal which is transmitted via the cable is measured in the antenna transmitting/receiving unit before and after amplification of the radio signal, the additional measurement result is transmitted via the cable to the transmitting receiving unit. The additionally measured level is transmitted via the cable to the transmitting/receiving unit, and a further level difference between the measured levels is determined. If there is any discrepancy in the further level difference, this is included in the influence on the transmitted signal. This measure also allows different gain factors, caused by severe temperature fluctuations, of the antenna transmitting/receiving unit to be detected and to be compensated for by including the measurement result in the transmitted signal level control.
The measurements of the level of the transmitted signal are advantageously carried out continuously or at regular time intervals. Continuous measurement of the level allows detailed control of the level while, in contrast, the measurement of time intervals means a reduced dynamic load on a microprocessor which is used for the method according to the present invention. In the case of measurement at regular time intervals, there is no need for integration of the measured values, via which hectic control action is smoothed out. In the case of continuous measurement, the measured level values of the level of the transmitted signal are advantageously integrated.
The levels are measured as voltage or power levels and the level difference is determined as a power level difference, with the level being converted to a power level if the voltage level is measured.
According to a further advantageous embodiment of the method of the present invention, the transmitted signal is input, and the received signal which is received with the aid of the antenna is output in the transmitting/receiving unit with the aid of a diplexer which is connected to the cable, with the transmitted signal being passed via a controllable attenuater and an amplifier to the diplexer. The transmitted signal is output and the received signal is input in the antenna transmitting/receiving unit with the aid of a diplexer which is connected to the cable, The transmitted signal is passed via an antenna transmission amplifier and the received radio signal is passed via an antenna reception amplifier. The transmitted signal is input and the received signal is output in the antenna transmitting/receiving unit with the aid of a further diplexer which is connected to the antenna cable. The level of the transmitted signal is, in each case, measured with the aid of level meters which are arranged in both the antenna transmitting/receiving unit and in the transmitting/receiving unit, and a setting signal is calculated in the transmitting/receiving unit, with the aid of a microprocessor, in order to set the attenuater. The advantageous control of the transmitted signal level with the aid of a controllable attenuater allows level control to be achieved over a wide level range.
In an installation of the transmitting/receiving unit configuration explained above, the level of the transmitted signal is measured, and the level difference is determined in both the antenna transmitting/receiving unit and the transmitting/receiving unit, and the level difference is set with the aid of the attenuater in such a manner that a level which is dependent on the transmission method is measured in the antenna transmitting/receiving unit. This level difference which is set is kept approximately constant during continuous operation with the aid of the attenuater. Alternatively, the variable attenuator can be set to a basic setting by the measured level difference between the two transmitting/receiving units and, in the course of operation, the level difference of the transmitted signal is measured before and after amplification in the antenna transmitting/receiving unit, wherein the attenuater is set as appropriate for the level difference. This assumes that, once the transmitting/receiving unit configuration has been started up, the transmission characteristics of the cable vary less than those of the temperature-dependent antenna transmission amplifier, that is, considerably greater level fluctuations are caused by the antenna transmission amplifier. In addition, the level difference caused by the cable should be determined at relatively long time intervals, and the variable attenuater should be set as appropriate for the level difference discrepancies which are determined.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Preferred Embodiments and the Drawings.